There are many known stand up pouch machines, and this invention will be explained in the context of pouch machines such as the CMD® Stand-Up Pouch Machine® or that shown in U.S. Pat. Nos. 6,976,946, 7,191,575 and 7,3253,79, each of which are hereby incorporated by reference, and US Patent Publications US-2014-0332138-A1 and US-2011-0207589-A1, each of which are also hereby incorporated by reference. One example of a prior art pouch machine with a zipper crush section includes an unwind or infeed section, followed by a forming or folding section, followed by a machine direction bottom seal and zipper flange sealing section, followed by a zipper crush section, followed by a side sealing section, followed by other processes such as cut off, hole punch, etc. The film is advanced and stopped, and each section operates on the portion of the film in that section. Then, when the operations are complete, the film is advanced, and each section again operates. When the film has completed the path through the machine each operations has been performed on the film, thus creating the pouch.
Pouch design have become complex, such as having any of a number of features, including closable zippers, easy open tear strings, product protecting venting, etc. These features are added with insert material or materials that are different than and/or have a different thickness than the pouch body or substrate material. Insert, as used herein, refers to a feature such as a closure (zipper, hook and loop, press to close, etc), a tear strip, a vent, plugs, valves, spouts, strips, etc. that are comprised of materials different than and/or have a different thickness than the materials comprising the pouch body or substrate.
An insert, such as a zipper, is typically provided as a continuous strip (or strips) which must be compressed (crushed) and sealed to the pouch. The portion of the zipper near the edge of the pouch must be sealed to properly seal the pouch. The prior art machines were costly to operate and often made poor seals around the insert and/or took too much time.
It is difficult to crush and seal inserts to the substrate because the insert or zipper is thicker than the pouch, and comprised of a different material. Creating consistently strong seals requires the sealing pressure be uniform across the length of the seal and the additional thickness of the insert material causes inconsistent pressure in the seal area.
The prior art typically sealed the edge of the zipper/pouch by either crushing it with a modified sealer (called a zipper crush or zipper crush sealer), or heating it with ultrasound. Crushing the zipper minimizes the seal pressure differences along the length of the side seal. If inserts are not sufficiently crushed there will be insufficient pressure to create a seal between the pouch substrates were it intersects the insert material FIG. 1 shows a cross sectional view of a zipper insert 102 inserted between two layers 104 and 106 of a pouch substrate. FIG. 2 shows the zipper insert and substrates after crushing.
Crushing the insert in one step directly from its original thickness to the final thickness makes it more likely for the seals to be fractured. Thus, prior art machines sometimes included two separate sealers/crushers. The first sealers/crusher crushed the zipper most of the way, and the second sealers/crusher crushed the zipper to the final desired thickness. Even with two conventional crushing stations, pouch converters needed to run at slower than desired speeds to create strong seals with zipper material because of fractured seals.
Crushing and sealing the insert requires a heat source, pressure, and time. The heat source must have enough thermal mass to melt the insert material and enough rigidity to maintain its shape even with significant crush pressure. A hot bar (sometimes called a seal head or platen) is typically used in the prior art. Most prior art pouch machines use continuous pressure systems to crush insert materials. These crushing unit are typically adaptations of a cross seal head station. Pneumatic cylinders, or springs are used to create the continuous pressure (either constant or gradually changing), and utilize heaters and tooling smaller than the cross sealers. This method is effective to flatten the insert but can create fractured seals and/or poor bond strength between the insert and substrate and/or between the insert materials. Seal heads can be driven with a servomotor and eccentric linkage, but this approach is more expensive than continuous pressure systems.
Prior art continuous crush pressure systems do not always provide consistent seal bond strengths. Crushing pressure is nominally constant with slight variations and is uncontrolled, open loop, and the continuous crushing force creates and maintains shear and stain stress. Shear stress is a force perpendicular to the crushing force and parallel to the plane created by difference in flow rates. Strain is created by intermolecular mechanical obstruction and friction, and is dependent on pressure and temperature.
Crush rate and final head position are determine by equilibrium—when the crushing pressure equals the shear resistance of material the head will stop. Therefore seal shear stress and intermolecular stress are considerable higher than desirable. Crystalline sections of the polymer chain may not uncoil and relax into their new position because the crushing force adds more intermolecular stain. If the amorphous sections are orientated in a direction not conducive to entangling, the crushing force will maintain strain and prevent further entanglement. This means oriented amorphous sections are unable to properly entangle due to the continuous crush pressure and stress it creates, and the crystalline section can spring back causing shear on the seal.
Prior art crush systems are limited in performance and cannot be optimized for seal strength and run rate. Running lower crush pressure will improve seals, but will require longer cycle time, which can limit machine run rates. Cycle rate can be minimized with higher pressures, but at the cost of higher shear and stain, and lower seal bond strength.
Accordingly, a machine and method to make pouches with inserts or make bags with seals that can consistently and economically create quality seals in a timely fashion without creating undue seal shear stress and intermolecular stain, is desired.